1. Field of the Invention
The present invention relates to a process for the removal of selenium from aqueous media.
2. Description of the Prior Art
Selenium is a naturally occurring group VIA element found in rocks and soil with the average concentration in the earth's crust being about 0.7 ppm. Small amounts of selenium are commonly found in air, water, plants, crops and fossil fuels. It exists in several oxidation states, including selenide (-2), elemental selenium (0), selenite (+4), or selenate (+6). While all of these oxidation states can be found in nature, selenite and selenate are the more common soluble forms found in water systems.
Processes such as petroleum refining and agricultural irrigation often result in wastewaters with concentrations of selenium which are too high to be released into public waters due to safety concerns. While trace amounts of selenium are necessary for life, and in some instances have been found useful for the treatment of various diseases, concentrations of soluble selenium greater than those necessary to sustain life or provide treatment of diseases are potentially harmful to humans, birds, livestock and agriculture. Concerns over the toxic effects of selenium have prompted an increasing need to efficiently remove selenium from industrial media before the wastewaters are reintroduced into the environment. Currently, only a limited number of methods for removal are available.
Prior wastewater treatment techniques have been used to remove relatively small levels of selenium. These techniques have included lime precipitation, reverse osmosis techniques and various adsorption techniques such as iron and aluminum hydroxide adsorption/coprecipitation and activated carbon adsorption.
Of these techniques, the use of lime precipitation has not achieved adequate technical success and reverse osmosis has proven to be very costly for the treatment of large volumes of media. The efficiency of selenium removal using the various adsorption techniques is partially dependant upon the form or oxidation state of the selenium present, as well as the competitive ions in the media. These mixed forms or oxidation states of selenium can potentially hamper removal within a single treatment method. Because most waters include a mixture of selenium species, it has been difficult to approach complete removal using only one step. The result has been the need to convert the selenium present into the same or substantially similar forms prior to attempting removal or to treat the waste stream where the selenium is found in a chemically similar form. These multiple step methods are often costly and have proven difficult to implement.
U.S. Pat. No. 4,405,464, issued Sep. 20, 1983, discloses a process for reducing selenium in the Se(VI) oxidation state by mixing the aqueous solution with a quantity of metallic iron. The iron then reduces the selenium and is collected by precipitation.
U.S. Pat. No. 4,806,264, issued Feb. 21, 1989, discloses a process for removing selenium ions from an aqueous solution by reducing the selenium ions with an amount of ferrous ions to elemental selenium.
Still another method, disclosed in U.S. Pat. No. 4,915,928, issued Apr. 10, 1990, involves removing selenium from wastewaters using strong-base anion exchange resins. This process works only after sulfur, which is often present in higher concentrations than the selenium, has been stripped from the wastewaters.
"Chemical Process for Removing Selenium From Water," U.S. patent application PB91-155382, filed by the Department of the Interior Nov. 9, 1990 and available through the National Technical Information Service, discloses a process for the removal of organoselenium compounds and selenate using a combination of a transition metal and an electropositive metal.
A process has now been discovered in which selenium in its various oxidation forms may be removed from aqueous media by utilizing a porous support having metal ions selected from the group consisting of transition metal ions, alkaline earth metal ions and mixtures thereof either complexed or exchanged thereon. One advantage of this process is that it provides an environmentally compatible means to dispose of or recover complexed or bound selenium, especially with regard to the removal of selenium from sour water produced by petroleum refining.